The present invention relates to a shredder for cutting of wastepaper into small pieces or shreds.
Known shredders generally include a casing having an inlet opening for receiving material to be shredded, such as documents, and an outlet opening for discharging the shredded material, a shredding mechanism composed of a motor, a speed reducer and a rotary cutter, and a switch means for changing directions of rotation of the motor selectively to actuate the rotary cutter to cut off the material or release the rotary cutter from locking or biting engagement with the material being shredded.
The switch means is operated to unlock the rotary cutter, when the rotary cutter, while in operation, is subjected to an undue load and locked against rotation because, for example, an increased number of sheets of paper are supplied to the rotary cutter. More specifically, if the rotary cutter remained locked while being actuated, the motor for driving the rotary cutter would continue to be excessively loaded and tend to be burned out or otherwise damaged. When the rotary cutter is locked against rotation, therefore, the switch means should immediately be actuated to de-energize the motor and then to reverse the motor and the rotary cutter to displace the material out of locking engagement with the rotary cutter and to return the material to the inlet opening. The switch means in the conventional shredder has a switch for rotating the motor in a normal direction and another switch for rotating the motor in a reverse direction. In operation, one or the other of the switches is operated at any one time to start the motor and the rotary cutter or to change the direction of rotation of the same.
When the rotary cutter is to be reversed in rotation so as to be unlocked out of the material being shredded, the motor is required to produce a torque large enough to release the rotary cutter from biting engagement with the material and to force the freed material through a passage back to the inlet opening. The force thus required to release the rotary cutter and to move back the material is imposed on the motor as a load during reverse rotation of the rotary cutter.
However, the motor in prior shredders is designed to produce a constant torque irrespective of whether the motor is rotated in the normal or the reverse direction. Such a motor is likely to fail to free or unlock the rotary cutter when the material is jammed in the rotary cutter, and hence tends to be placed continuously under an undue load. Unlocking the rotary cutter thus requires temporary de-energization of the motor and manual removal of the material out of locking engagement with the rotary cutter. The manual material removing operation, however, is laborious, tedious and time-consuming.
With the conventional shredders, the motor is temporarily de-energized and then rotated in the reverse direction when the reverse rotation switch is turned on to release the rotary cutter. The reverse rotation switch is constructed such that it allows continuous rapid movement of a switch contact from a motor de-energization position to a reverse rotation position without experiencing a time delay. When the reverse rotation switch is actuated to move the contact to the reverse rotation position speedily in a single stroke, the motor while rotating in the normal direction is subjected to an abrupt force tending to rotate the motor in the opposite direction, whereupon the motor and the speed reducer undergoes damaging stresses. At times, electrical circuits for rotating the motor in the normal and reverse directions are simultaneously short-circuited.